1. Field of the Invention
The present invention relates to audio circuits and amplifiers, and more particularly to a true differential microphone amplifier that amplifies weak signals from a microphone while rejecting common mode noise.
2. Description of the Related Art
Many handheld and portable consumer electronic devices, such as cell phones, personal digital assistants (PDAs), digital cameras, laptop computers, etc., use a microphone. A common microphone configuration for such electronic devices comprises an electret condenser having a diaphragm which performs sound to voltage conversion. The electret condenser is coupled to the gate of a junction field-effect transistor (JFET) device used as a buffer inside the microphone. The gate of the JFET detects audio signals converted by the electret condenser and the drain and source of the JFET are used as the two-wire interface to an amplifier or preamplifier to amplify the audio signal provided as a current signal. Although the present invention is illustrated for an electret condenser type microphone, other types of signal generating devices are contemplated.
Some amplifier solutions are single-ended in which one terminal of the two-wire interface of the microphone is coupled to ground. Other solutions that do not ground a microphone pin often include a “microphone bias” pin, which increases the pin count on the preamplifier integrated circuit (IC) or chip. Most solutions require external support devices, such as bias resistors and/or coupling capacitors. Differential voltage solutions are known but suffer from distortion problems. Current mode solutions are known but are usually single-ended.
It is desired to provide a cost effective amplifier circuit for the microphone which meets noise, distortion and power supply rejection (PSR) specifications for electronic devices. It is desired to provide a solution that amplifies weak signals from the microphone while rejecting the common mode noise. It is desired to provide a differential solution that connects directly to the two interface terminals of the JFET microphone without the need for external components.